126 THE THIRD DAY. [CHAP. 



a crest attached to the roof of the brain, while its two 

 laminae become fused. 



Anteriorly, the neural crest extends as far as the 

 roof of the mid-brain. The pairs of nerves which 

 undoubtedly grow out from it are the fifth pair, the 

 seventh and auditory (as a single root), the glosso- 

 pharyngeal and the various elements of the vagus (as a 

 single root). 



After the roots of these nerves have become estab- 

 lished, the crest connecting them becomes partially 

 obliterated. The roots themselves grow centrifugaUy, 

 and eventually give rise to the whole of each of the 

 cranial nerves. Each complete root develops a gan- 

 glionic enlargement near its base, and (with the ex- 

 ception of the third nerve) is distributed to one of the 

 visceral arches, of which we shall say more hereafter. 

 The primitive attachment of the nerves is to the roof 

 of the brain, but in most instances this attachment is 

 replaced by a secondary attachment to the sides or 

 floor. 



The rudiments of four cranial nerves, of which two 

 lie in front of and two behind the auditory vesicle, 

 are easily seen during the third day at the sides of the 

 hind-brain. They form a series of four small opaque 

 masses, somewhat pearshaped, with the stalk directed 

 away from the middle line. 



The most anterior of these is the rudiment of the 

 fifth nerve (Figs. 42 and 67, Y). Its narrowed outer 

 portion or stalk divides into two bands or nerves. Of 

 these one passing towards the eye terminates at present 

 in the immediate neighbourhood of that organ. The 

 other branch (the rudiment of the inferior maxillary 



